Rimfire firearms are some of the most popular firearms worldwide. The term rimfire refers to the type of cartridge used by these firearms. The cartridges are referred to as rimfire because the firing pin of a firearm strikes and crushes the base's rim to ignite the primer. This is in contrast to the more common centerfire cartridges where the firing pin strikes the primer cap at the center of the base of the cartridge.
The rim of the rimfire cartridge is essentially an extended and widened percussion cap which contains the priming compound, while the cartridge case itself contains the propellant powder and the projectile (bullet). Once the rim of the cartridge has been struck and the bullet discharged, the cartridge cannot be reloaded, because the head has been deformed by the impact of the firing pin.
Rimfire cartridges are limited to low pressures because they require a thin case so that the firing pin can crush the rim and ignite the primer. Rimfire calibers up to .44 (11 mm) were once common when black powder was used as a propellant. However, modern rimfire cartridges use smokeless powder which generates much higher pressures and tend to be of .22 caliber (5.5 mm) or smaller. The low pressures necessitated by the rimfire design mean that rimfire firearms can be very light and inexpensive, which has helped lead to the continuing popularity of these small-caliber firearms.